The color processing systems of prior imaging equipments, such as color TVs or computer monitors originally use a static white balance as the standard to determine the proportion of primary colors. According to the standard stipulation, pick-up cameras use a standard white of D65 with a correlated color-temperature of 6500K. Theoretically, in order to avoid the distortion of color reproduction, displaying equipments should also use a standard white of D65. However, products designed and calibrated according to this standard have the following problems: objects with white or negative colors do not have satisfying visual effects; green plants look withered; sky appears gray and dark. Customers are not satisfied with the total effects with bad clarity and faded images.
In order to resolve above problems, manufacturers put forward many methods. According to the references searched and the practice by manufacturers, there are mainly several technologies designed and applied as follows:
Calibration aiming at some certain colors that are extremely unsatisfactory. For example, in 1976 RCA company announced an integrated circuit for automatic adjusting of complexion in the journal of IEEE Transaction of Consumer Electronics. The circuit allowed viewers manually rectify complexion through the adjusting of the phase of color sub-carrier. This circuit was not popularized. SONY company's European patent (patent number EP0172754) brought forward a “color calibrating circuit”. Its main point lies in the emphasis on the blue color in white signals, which makes white objects look beautiful. This circuit is applied in the top-grade products of SONY company and Panasonic company.
To calibrate white balance as much as possible. Some calibrations also give consideration to the effects of environmental light. The examples are the U.S. Pat. No. 4,709,262, SONY company's European patent numbered GB2149267 and Japanese patent numbered from JP62268289 to JP62268292, Fuji company's European patent numbered EP322791 and some recent automatic white balance adjusting circuits using bus-mastering.
A popular method now is that setting a white reference which may deviate from the accurate chrominance value, but deliver a more satisfying viewing effect. In this case, all images are partial to the same color hue. Nearly all the existing products use a white reference that is different from the one used by pick-up equipments. Most of color TVs and computer monitors use a static white reference between 9300K and 11500K as white balance. This kind of technical proposal makes all the images in a status of chrominance distortion. This distortion can improve the effects of color reproduction for white or some colorful objects, but at the same time obviously degrade the effects of color reproduction for some other colors. And this degradation is an unacceptable inferior chrominance distortion for some images. For instance, when using some white reference with high color-temperature, say 9300K, the percentages of green primary-color and blue primary-color are much larger than their accurate values in the images displayed by the equipments. Thus, complexion looks yellow or dark green and objects with warm colors, such as flowers, clothes, vessels and buildings lose their original vivid colors.
At the present time, however, most manufacturers and users abandoned the standard white of D65 that does not introduce distortion. Instead, they turned to such standard with higher color-temperature, which causes distortion for all the colors. The reason lies in that the latter gives more images with better viewing effects than the former does, though it's far from being perfect. The development mentioned above shows that the best reproduction of colors is not definitely the one that never introduces any distortion in the measurement of chrominance, but the one that is full of genuine and beautiful images in terms of viewing effects. The problems in pursuing the best reproduction of colors are not just the problems about chrominance. The different combinations of color hue, chroma and relative brightness bring different feelings to people. In addition to physical stimulation, the factors impacting feelings of colors also include psychological and physiological mechanisms. For instance, the feeling of truthfulness is affected by contrast effect of apposition colors and also by whether or not the colors of the images are matching the colors of people's memory. And the feeling of beautifulness is affected by the color-preference law. Thus, the best effects can only be acquired by combination of physical factors and non-physical factors. Prior arts give no consideration to the non-physical factors mentioned above. Therefore, viewers do not definitely feel best even for many color images that are exactly reproduced in terms of chrominance measurement. There is another kind of product allowing users to manually adjust color hues by remote controllers or switch knobs. No matter which point it goes, the final effect is that the whole image is partial to the same color hue and not all the aspects can be taken care of.
Furthermore, prior art's capability to tolerate chrominance deviations is relatively limited. During the production and transmission of image signals, deviations are not avoidable. As mentioned above, existing technologies cannot make all images stay in the best reproducing status. Some reproduced objects even lie in the area of inferior chrominance distortion or around the limit of acceptable quality. Thus, the color effects get remarkably worse even if a small deviation of chrominance signal occurs. As a result, equipments'ability to tolerate deviations is not good.
In a word, after years of efforts by manufacturers, existing products still have the following disadvantages in common: the reproduction of all kinds of objects can not be accordingly taken care of in the images displayed; inferior chrominance distortion can not be completely eliminated; the ability to tolerate chrominance deviations is relatively limited.
The purpose of the present invention is to provide a method and apparatus for adaptive compensation of chrominance, eliminate inferior chrominance distortion and make all the images displayed by imaging equipments meet the requirements that the best color reproduction can be achieved automatically. At the same time, equipments'ability to tolerate chrominance deviations can be improved.
For the sake of convenience, we provide as follows some definitions and explanations:
The following symbols are used to indicate the output primary-color voltages and their proportion coefficients for the circuit that is sitting before video amplification circuits or matrix circuits in the original equipments: Er is voltage of red primary-color; Eg is voltage of green primary-color and Eb is voltage of blue primary-color. r is voltage proportion coefficient for red primary-color; g is voltage proportion coefficient for green primary-color and b is voltage proportion coefficient for blue primary-color, where r=Er/(Er+Eg+Eb), g=Eg/(Er+Eg+Eb) and b=Eb/(Er+Eg+Eb).
We construct rgb chromaticity diagram by setting r as abscissa and g as ordinate. Each point in rgb chromaticity diagram corresponds to one point in CIE chromaticity diagram. This relation can be acquired through calculation using chrominance formulas or by measurement. For instance, we can make a color TV with D65 white balance to display color images with chromaticity coordinates of (x,y). After measurement of Er, Eg and Eb, coordinates of (r,g) can then be calculated using above mentioned equations. Above voltage signals of primary colors, namely Er, Eg and Eb, correspond to point F in rgb chromaticity diagram (see FIG. 4) and also point F in CIE chromaticity diagram (see FIG. 3).
All the colors that can be reproduced by the imaging equipment correspond to a certain region in CIE chromaticity diagram and rgb chromaticity diagram. This region can be divided into a series of small regions, called color-gamut cells. If j represents the serial number of a color-gamut cell, then Uj indicates that cell. For each color-gamut cell, there is a color representing its chrominance characteristic. This color is called chrominance sample. The chrominance sample of color-gamut cell Uj is marked as (Scj). Chrominance sample (Scj) corresponds to point Scj in rgb chromaticity diagram (see FIG. 4) and also point Scj in CIE chromaticity diagram (see FIG. 3). The total number of color-gamut cells is N. At D65 white balance, the signal from the equipment that corresponds to chrominance sample (Scj) is called chrominance sample signal (Scj). After optimizing treatment, the chrominance sample signal (Scj) becomes a new imaging color, called optimized color of chrominance sample (SCEj).
We sample all the actual objects that can be represented by the colors of each color-gamut cell and these sampled objects are called actual-object samples. The signals obtained by picking up of these samples with camera are called signals of actual-object samples. The signals of actual-object samples belonging to the color-gamut cell Uj are input into the equipment. At D65 white balance, the image displayed by the equipment is called original image of actual-object sample, (SOj). If the equipment is using an optimized white balance DE and it is also in DE white balance status, the image displayed now is called common image of actual-object sample, (SOCj). If the equipment is in DE white balance status and the signals are given adaptive compensation, the image displayed is called optimized image among the actual-object samples, (SOEj).